Along with the development of packet and broadband of the telecommunication network, the Internet protocol (All-IP) has become the trend of the future service network evolution. According to the prediction, in the next five years, the bandwidth will increase with a growth rate of more than 50% per year. At present, no matter in the backbone level or in the metropolitan area level, the two-layer networking mode of the Wavelength Division Multiplexing (IP over WDM) is gradually replacing the original three-layer mode of the Synchronous Digital Hierarchy (IP over SDH) over WDM, and the flatten architecture of directly bearing IP on the optical layer has become a trend. The IP over WDM networking architecture proposes a new requirement to the optical layer WDM device, and the networking, service scheduling and end-to-end circuit monitoring management function originally done by the SDH network will be mainly charged by the WDM level gradually.
The fundamental driving force of the optical transport network, as the bearer plane of the IP bearer network, facing changes is from the requirements of the IP bearer network. At present, along with increasing of the size of the service granule carried by the IP bearer network, large granule services, such as 10 GE (Gigabyte Ethernet) service, continue to emerge, which will require that the WDM device has the function of flexibly scheduling large granule services. The whole networking idea required can be divided into a region scheduling and inter-region convergence and scheduling which is taken charge by an introduced device with electrical cross function. The core cross capability of electrical cross system is strong, the scheduling of electrical cross system is flexible, and the electrical cross system can achieve the flexible scheduling of 1 GE˜10 GE granules.
Facing the challenge of IP, the metropolitan area optical network device is required to have the flexible service scheduling capability. At present, the scheduling cross of services is implemented primarily by an optical cross based on the Reconfigurable Optical Add-drop Multiplexer (ROADM) and an electric cross based on the Optical Transport Network (OTN). For the solution of the electrical cross, the network nodes can be achieved with the centralized or distributed electrical cross system. The centralized electrical cross system adopts special client-side single-boards and line-side single-boards, and uses a cross single-board to schedule the services; and the trunk service transmission is achieved in the line-side single-board, and the add/drop path of tributary service is achieved in the client-side single-board. The single-boards in the centralized electrical cross system have definite division, and the system function is relatively perfect. Since the centralized cross unit takes a sub-frame as a unit, the resource allocation is designed according to the largest cross capability of the sub-frame; and in the case that the access service capacity is suitable to the design capacity, the performance and price ratio is relatively high, while in the case that the services that need to be scheduled are few, there is resource waste. Therefore, the backbone network and the core network generally adopt the centralized electrical cross system for service scheduling, thus ensuring the network stability.
The distributed electrical cross system is characterized in that there is no independent cross unit, and the cross function is achieved by the fixed connection between the electrical cross unit within each single-board in the distributed electrical cross group and each single-board. Due to the constraint on the fixed connection complexity of the backboard of the network node, the distributed electrical cross group generally takes 4 single-boards as a group, and if there are more than 4 single-boards, it will result in dramatic increase in the circuit connection complexity. Compared with the centralized electrical cross system, the distributed electrical cross has a significant reduction in cost, therefore, in the case that the edge nodes, etc., in the metropolitan area network have not enough processing capability, the distributed electrical cross system is usually adopted.
The existing distributed electrical cross system only is generally a simplified version of the centralized cross system, it is only improved on the basis of the client-side single-boards and the line-side single-boards in the existing centralized cross system to integrate the cross units into the single-boards rather than to centralize to implement through the cross board. Since the single-boards are not improved according to the feature of distributed cross, each single-board only achieves the client-side or line-side service; if the line-side service is protected, the add/drop path of the client-side service cannot be realized, and the sub-wavelength level service cannot be protected also, thus resulting in that the distributed cross scheduling and protection function is limited.
The original ITU-T G808.1 standards respectively defines the sub-network connection (SNC) cascade protection and the SNC cascade protection with the device protection, which is mainly achieved based on the protection switching of the optical cross system or can be achieved through the electrical cross connection.
FIG. 1 is a schematic diagram of a sub-network service scheduling and protection Sublayer (SNC/S) cascade system. As shown in FIG. 1, two network layer trail endpoints E connect with the protection switching function nodes P through the connection points C, and four sub-networks are single-point connection through two protection switching function nodes P in the network nodes. When a service in one working sub-network has fault, this service can be replaced with the concurrent service in the protection sub-network to ensure that other places in the network are not affected; however, when any one of the protection switching nodes has a fault, the entire network necessarily faces to paralysis.
If the existing network nodes directly use the optical cross system, although it is able to perform the flexibly scheduling and protecting to the client-side service and the line-side service, it is not suitable to the case that the edge nodes in the metropolitan area network do not have enough processing capability due to the high costs of optical switches, optical-electrical converters and optical cross device. Moreover, the optical cross system cannot achieve the SNC cascade scheduling and protection of the sub-wavelength level service.